1. Field of the Invention
The present invention relates to an end mill used for machine tools such as milling machines and machining centers.
2. Prior Art
An end mill shown in FIGS. 1 to 3 (hereafter referred to as "conventional tool 1") is used as a rotary cutting tool when cutting steel and other general materials by using machine tools such as milling machines and machining centers. The conventional tool 1 is one of the most generally used conventional end mills. The peripheral cutting edges 2 of this conventional tool 1 have a positive rake angle .theta.b and the core diameter 4 of the cutting edge section is set to about 60% of the tool diameter to dispose wide chip pockets and to enhance the cutting performance of the tool.
As end mills used to cut workpiece materials having a hardness exceeding H.sub.RC 50, such as tool steel, an end mill shown in FIGS. 4 and 5 (hereafter referred to as "conventional tool 2") was proposed in the Laid-open Patent Application No. 3-178714, an end mill shown in FIGS. 6 and 7 (hereafter referred to as "conventional tool 3") was proposed in the Laid-open Patent Application Nos. 2-100727 and 3-26413 by the applicants of the present invention, and an end mill shown in FIGS. 8 and 9 (hereafter referred to as "conventional tool 4") was proposed in the Laid-open Patent Application No. 4-159010.
In particular, the conventional tools 2 and 3 can cut workpiece materials having a hardness of H.sub.RC 60 or more. These conventional tools have a large negative rake angle .theta.b of -60.degree. to -30.degree. and a large relief angle .theta.c on their peripheral cutting edges. The conventional tool 4 has a negative radial rake angle of -22.degree. to -5.degree. at the curved sections of the end-cutting edges thereof to enhance the strength of the end-cutting edges when cutting workpiece materials having a hardness exceeding H.sub.RC 50 or more. In FIGS. 1 to 7, numeral 1 represents a tool, numeral 3 represents a tool shank and numeral 5 represents a chip pocket.
Since working hours and man power are desired to be saved and workpieces are requested to have higher accuracy these days, NC machine tools such as machining centers have been used widely and mainly. For this reason, the demand for tools capable of ensuring reliable, highly accurate and efficient machining operation is increasing significantly.
In the case of the conventional tool 1, however, the cutting edge strength and the stiffness of the tool itself are insufficient to satisfy the above-mentioned demands. When the tool is used for high-speed, high-feed cutting, intense vibration generates and causes chipping and breakage, resulting in improper machined surfaces. The tool must thus be used in limited cutting conditions and it has low cutting efficiency. Even during normal cutting, sharp edges made of cemented carbide (inherently brittle material) attached to the conventional tool 1 still need to be improved with respect to tool stability.
When cutting workpieces having a hardness exceeding H.sub.RC 50, the conventional tool 1 wears out significantly, and is apt to be chipped and broken very easily in the same way as in the case of the above-mentioned high-speed, high-feed cutting. In order to cut hard workpieces, the conventional tools 2 and 3 are available. Since the peripheral cutting edges 2 of these conventional tools have a large negative rake angle .theta.b to satisfy the requirements for cutting hard workpieces, these tools are inferior in cutting performance and have great cutting resistance. In actual practice, these tools are applicable only to cutting operations which require slight depths of cut, such as finish cutting. In the case of the conventional tool 4, the length of its curved section is only 0.1 to 0.5 mm, making its chip pocket shallow. This tool is therefore applicable only to cutting operations which require slight depths of cut and cannot be used for general purposes.
When cutting workpieces having low hardness values, such as machine structural carbon steel, the effects of the characteristics of such workpieces on cutting tools become significant. The conventional tools 2 and 3 have low cutting efficiency and generate plucks on machined surfaces, causing a problem of very inferior cutting accuracy. Although the conventional tool 4 is also inferior in cutting efficiency, since its negative rake angle is smaller than those of the conventional tools 2 and 3, the surfaces machined by the conventional tool 4 are less affected by plucks. However, since its end-cutting edges are classified into a master tooth and a slave tooth, the strength of the slave tooth is reduced and the tool is apt to be chipped at its corners when cutting is performed under a heavy load.
As described above, the conventional tools 1 to 4, which have been made by more sufficiently delivering the characteristics of cemented carbide, are apt to cause problems of chipping and breakage when their cutting edges are sharp. When their cutting edges are obtuse, the tools are mainly used for finish cutting. When the rake angle is an intermediate value between the sharp and obtuse angles, that is, .theta.b=-20.degree. to 0.degree., the characteristics of both the sharp and obtuse angles are intermixed and delivered. The tools having such intermediate edge angles can be used for various applications, and can satisfy various accuracy and performance requirements. In actual practice, however, these matters have not yet been examined sufficiently.